1. Field of the Invention
This invention relates to a resin used for an optical material such as a plastic lens, a prism, an optical fiber, a substrate for an information recording medium, a filter and a light emitting diode, a polymerizable composition used as a raw material for the resin, and a novel polythiol compound with a higher sulfur content constituting the polymerizable composition. Particularly, these may be suitably used for an eyeglass with a higher refractive index. This invention also relates to a novel process for preparing a thiol with higher sulfur content. A thiol compound prepared by the process, in particular a polythiol compound, may be used as a raw material for preparing an episulfide compound or a resin used for an optical material such as a plastic lens, a prism, an optical fiber, a substrate for an information recording medium, a filter and a light emitting diode. Particularly, it may be suitably used for an eyeglass with a higher refractive index.
2. Description of the Prior Art
A plastic lens is lighter and less brittle than an inorganic lens, and dyeable, which has been therefore rapidly prevailing in the fields of optical devices such as a lens of eyeglasses and a camera lens. Such a plastic lens has been required to exhibit higher performance: e.g., a higher refractive index, a higher Abbe number, a lower specific gravity and improved heat resistance. Various resin materials for a lens have been developed for practical use. Among those, plastic lenses made of a polythiourethane resin proposed by us have been widely used, which have been disclosed in JP-As 60-199016, 62-267316 and 63-46213.
We have further achieved improvement of a refractive index by increasing a sulfur content in a thiol used for a polythiourethane as described in JP-As 2-270859 and 7-252207.
Since further improvement in a refractive index has been, however, required, an episulfide resin has been proposed (JP-As 9-110979, 9-71580, 9-255781 and 10-298287.
According to the processes, a higher refractive index can be achieved while maintaining a relatively higher Abbe number. However, although a polythiourethane resin from a conventional thiol compound may improve a refractive index to some degree, it may have a deficiency of reduced heat resistance which is one of important physical properties, depending on the type of the thiol used. Meanwhile, it has been found that an episulfide resin is less impact-resistant than a polythiourethane resin.
Thus, there has been a need for improved balance among a refractive index, heat resistance and impact resistance in a conventional resin with a higher refractive index.
There have been various attempts to prepare a thiol compound with higher sulfur content for a higher refractive index in a polythiourethane resin. The goal has not been achieved because a useful preparation process has not been established.
That is, it is necessary to develop a method for easily preparing a thiol compound with higher sulfur content.
An objective of this invention is to provide a polythiol compound which can provide an optical material made of a polythiourethane resin with improved balance among a refractive index, heat resistance and impact resistance.
Another objective of this invention is to provide a resin composition which can provide a polythiourethane resin with improved balance among the above properties.
A further objective of this invention is to provide a polyurethane resin with improved balance among the above properties, which is made of the above resin composition, as well as an optical material made of the resin, particularly a plastic lens.
To solve the problems, we have intensely conducted investigation. Finally, we have developed a novel polythiol compound and a polymerizable composition comprising the compound and have solved all the problems using a resin prepared by the polymerizable composition.
This invention relates to a polythiol compound represented by the general formula (1):
Rxe2x80x94(SCH2SH)nxe2x80x83xe2x80x83(1)
where R is an organic residue except for an aromatic group and n is an integer of at least 1, having at least two intramolecular mercapto groups and at least one dithioacetal skeleton represented by the following formula: 
where R1 and R2 are each independently a hydrogen atom or an organic residue except for an aromatic group, which may contain a mercapto group; R3 and R4 are each independently an organic residue except for an aromatic group, which may contain a mercapto group; R1 and R2 are not simultaneously a hydrogen atom; and R1 and R2, R1 and R3 or R4, R2 and R3 or R4, and R3 and R4 may be bonded to form a ring.
According to this invention, a resin with a higher refractive index, higher heat resistance and higher impact resistance with good balance may be provided.